(1) Field of the Invention
The invention relates to illuminated information displays and, more particularly, to an improvement in information displays utilizing small light sources such as light-emitting diodes or fiber optic wave guides.
(2) Description of the Prior Art
Referring to FIG. 1, a typical prior art LED display 10 utilizes a vertical cavity 11 defined by walls 12 that channel light from an electroluminescent semiconductor or LED 13 to an optical opening 14. The opening forms at least part of an information symbol such as an alphabetic character or a numerical character.
Unfortunately, the illumination intensity of commonly used LED's, such as GaAs.sub.1-x P.sub.x (gallium aresenide phosphide), is directional in nature. FIG. 2 shows a schematic representation of the relative intensity of light emitted from an upward-emitting LED 13 as a function of direction. LED's such as GaAs.sub.1-x P.sub.x approximate a Lambertian radiation pattern in which luminous intensity varies as the cosine of the off-axis angle .theta.: I.sub..theta. = I.sub.O cos .theta., where I.sub.O is the on-axis intensity, i.e., the luminous intensity parallel to the axis, y, which is perpendicular to the LED upper surface 15. (For LED's such as GaP (x = 1; gallium phosphide), the emission pattern can deviate from the Lambertian pattern, but exhibits an even stronger directional property.) The on-axis intensity I.sub.O (.theta. = 0.degree. ) is a maximum and drops off rapidly with increasing divergence from perpendicularity (that is, as .theta. increases). Thus, at angles of about 60.degree. relative to the LED upper surface 15, the intensity is less than 0.5 I.sub.O, and at about 75.degree. to 80.degree., the illumination is practically nil. In short, the emitted light is directional, the greatest intensity is in the direction perpendicular to the LED light-emitting surface 15, and almost all of the emitted light is concentrated within about 60.degree. of the perpendicular to the light-emitting surface.
Furthermore, for a typical flat optical opening, such as opening 14 in FIG. 1, the luminous intensity at a point in the opening can be approximated by I.sub..theta. = I.sub.O cos.sup.2 .theta.. Because of this cos.sup.2 .theta. dependency, the illumination across the optical opening 14 is non-uniform and the central area of the opening, particularly center 16 thereof, tends to be much more brightly illuminated than the edges 17--17.
Several approaches have been undertaken in the prior art to increase the uniformity of illumination provided by LED displays. One approach is to make the walls 12 specularly reflective and to provide a randomly refracting surface (not shown), such as a fly's-eye lens, across the optical opening 14. However, this approach typically is incapable of completely eliminating the non-uniformity of illumination and requires an undesirably thick cavity 11.
The uniformity of illumination can also be increased by altering the dimensions of the LED cavity, but this also can result in a too-thick cavity. The light flux density along the opening 14 varies from I.sub.O down to I.sub.O cos.sup.2 .alpha., where .theta.=.alpha., the angle subtended by lines directed from the LED surface 15 to the center 16 (axis "y" in FIG. 2) and to the edge 17 of the optical opening. As discussed above, the magnitude of the cosine.sup.2 term and the magnitude of the light flux density decrease rapidly as .theta. increases. The angle .alpha. can be decreased by decreasing the width, w, of the opening 14 or by increasing the thickness, t, of the cavity 11. Decreasing the width, w, decreases the size and visibility of the display information. Increasing the thickness, then, may be the only way to enhance uniformity of illumination, but frequently results in an undesirably thick cavity.
Another approach is taught in U.S. Pat. No. 3,780,357, issued Dec. 18, 1973 to Haitz. Here, a transparent media containing light scattering particles (not shown) is introduced into the display cavity 11 to enhance diffusion. This approach utilizing light scattering centers increases uniformity, but the repeated refraction/reflection of the light reduces maximum intensity. Also, as will be readily appreciated, for a given concentration of scattering centers, the amount of scattering and, thus, the uniformity of the intensity are enhanced by increasing the cavity thickness. Consequently, as in the previously described approach, the thickness is frequently undesirably large and increasing the thickness to optimize uniformity is done at the expense of the intensity.
Another shortcoming of the aforementioned and other prior art devices is that they utilize cumbersome connecting wires or leads that can increase the size of the display. This increased size is at least partially offset in the case of U.S. Pat. No. 3,876,900, issued Apr. 8, 1975 to Amatsuka et al., by using a side-emitting LED and a lateral, transparent, resin-filled channel having an optical opening in the upper surface. Connecting wires are attached to the top of the side-emitting LED and a light diffracting mask is positioned over the LED to enhance the uniformity of light emission. The wires and the mask do increase the thickness of the device, but considerable size reduction apparently results from the side-emitting, lateral-channel design.
However, the resin channel used by Amatsuka et al. tends to decrease the light transmission, at least in part because light emitted at large angles relative to the resin channel-air interface is not totally reflected into the channel, but instead is partially transmitted through the interface. The design requires a separation between the resin channel and the mask to avoid false display. That is, if a light-transmissive mask sags against the channel, the resin-air interface is eliminated and light is transmitted through the region of channel-mask contact. Also, the surface of the resin must be optically flat to avoid light leakage. Finally, the cost of manufacturing such a device is increased by the use of side-emitting LED's, which are more expensive than conventional, upward-emitting LED's, and by the use of an LED chip for each character segment.
As may be appreciated from the foregoing discussion, it is highly desirable to have a light-emitting diode display that provides uniform illumination using a thin structure that does not require the use of complicating and expensive elements such as masks or shields or light scattering media. Furthermore, since the LED chip itself is the most expensive component of the display, it would be advantageous to decrease the cost of the chip by decreasing the size and/or the number of chips used.